Manufacturers who sell both directly and via retail usually offer lower prices to retailers (and usually don't disclose those prices publicly), asking the retailers to sell at a suggested price (MSRP, often same as the direct sales price). So if you buy direct the manufacturer has a notably higher profit margin.

Big retailers can often get volume pricing which allows them to keep the prices even lower (if they're willing to reduce profit margins - and yes they do need good logistics to pull this off). Typically the retailers try to make up for the lower margin in volume of sales (and/or by convincing you to buy other higher margin stuff from them too), like when an electronics store selling you an iPhone tries to sell you insurance and accessories to increase their margin since that range of phones are low margin items.

Fraudulent parts on Amazon is a mix clever and just plain dumb I got bricks instead of a GPU. For work I’ve gotten a handful of hard to notice frauds. We got some Samsung SSDs the older sata drives not NVMe. One was a different shade of black and looked altered. It had the guts of a USB drive with a usb to sata homebrew conversion. That one Amazon took back no questions but it was a company purchase so they probably treat those differently.

For Legos if they were frauds I would imagine it would be noticeable. It’s probably a discount because of the volume they purchase.

If you buy legos often I’ve had some good luck going to goodwill stores and getting them super cheap. It’s not going to be a set. Usually it’s mixed pieces in a vacuum sealed bag or bin if there are a lot.